There have been many developments relating to chemical compositions for a variety of grades for carbon steels used in the production of rail products. The rail industry continually moves toward higher axle loads and higher speeds in an effort to increase track efficiency, which emphasizes the demand for improved performance of rail in tracks.
U.S. Pat. No. 5,658,400 describes a high carbon, pearlitic steel rail having high strength, wear resistance, ductility, and toughness that is manufactured by applying special rolling practice to produce fine-grain pearlite blocks in steel containing 0.6-1.2 mass % carbon, 0.1 to 1.20 mass % silicon, 0.40-1.50 mass % manganese and one or more elements selected as required from the group of chromium, molybdenum, vanadium, niobium and cobalt, thus imparting high wear resistance and an elongation of not less than 12% and a V-notch Charpy impact value of not lower than 25 J/cm2, in particular U.S. Pat. No. 5,658,400 indicates that manganese alloy levels below 0.40 mass % do not produce the desired effects.
U.S. Pat. No. 5,762,723, which was reissued as RE 42,668, describes a rail made from steel having improved wear resistance and damage resistance. The patent describes a rail made from a steel having a composition comprising more than 0.85-1.20 mass % of carbon, 0.10-1.00 mass % of silicon, 0.40-1.50 mass % of manganese, and if necessary, at least one member selected from the group consisting of chromium, molybdenum, vanadium, niobium, cobalt, and boron, and retaining high temperature of hot rolling or a steel rail heated to a high temperature for the purpose of heat treatment, to provide a pearlitic steel rail having good wear resistance and good damage resistance, and a method of producing the same, wherein a head portion of the steel rail is cooled at an accelerated rate of 1 to 10° C./sec from an austenite zone temperature to a cooling stop temperature of 700° C. to 500° C. so that the hardness of the head portion is at least 320 HV within the range of a 20 mm depth below the surface of the rail head. U.S. Pat. No. 5,762,723 also indicates that wear resistance generally increases (amount of mass loss due to wear generally decreases) with increasing hardness and decreasing pearlite interlamellar spacing.
Furthermore, U.S. Pat. No. 7,288,159 describes an improved steel for rails, and the methods for producing the same wherein the steel is described as having a carbon content in a range from more than 0.9-1.1 mass % where the high carbon steel rail is characterized as having a pearlitic structure. The average ultimate tensile strength is in a range from 204,860 to 222,120 psi with a minimum of 174,000 psi. The average yield strength was in a range from 132,320 to 148,450 psi with a minimum of 120,000 psi. Moreover, the method describes a fully pearlitic steel rail of high toughness and high wear resistance, consisting essentially of: forging a steel billet comprising the elements in a range from more than 0.9-1.1 mass % of carbon, 0.26-0.80 mass % of silicon, 0.8-1.2 mass % of manganese, less than or equal to 0.35 mass % of chromium, the balance of iron, and residual elements; hot rolling the billet to a rolling finishing temperature of about 1,000° C. and thereby forming a rail, and cooling the rail at a selected cooling rate in the range from 3.3 to 4.3° C./sec beginning substantially at said rolling finishing temperature and continuing at least until the pearlitic transformation completion temperature.
Also, U.S. Pat. No. 7,217,329 describes a steel railroad rail and methods for producing same, having a carbon content in a range from 0.7 to 0.95 mass %, a manganese content in a range from 0.8 to 1.2 mass %, and titanium content in the range of 0.005 to 0.105 mass % that has increased wear resistance and increased fracture toughness over conventional steel rail. The rail is characterized as having a pearlitic structure of a eutectoid nature. The average ultimate tensile strength is in a range from 178,000 to 207,000 psi, with a minimum of 174,000 psi. The average yield strength is in a range from 122,000 to 141,000 psi, with the minimum of 120,000 psi. The average total elongation is in a range from 10.3% to 12.5%, with a minimum of 10.00%. The Brinell hardness on the surface at any position of the head top and upper gage corners of the rail is in a range from 370 to 420 BHN. The hardness 19 mm below the top surface is in a range from 360 to 405 BHN and 19 mm below the surface at the upper gage corners is in a range from 360 to 410 BHN.
Additionally, U.S. Pat. No. 8,361,246 describes a pearlitic rail steel having a composition of 0.65-1.2 mass % of carbon, 0.05-2.00 mass % of silicon, 0.05-2.00 mass % of manganese and the balance composed of iron and inevitable impurities. The pearlitic rail is further specified to have a maximum surface roughness of 180 μm and a minimum ratio of the surface hardness to the maximum surface roughness of 3.5.
As another example, U.S. Pat. No. 8,469,284 describes a rail steel containing 95% pearlite structure below the surface of the rail, demonstrating a maximum manganese sulfide inclusion aspect ratio of 5 below the rail surface, and possessing a head hardness of 320-500 HV. The rail is composed principally of 0.65-1.2 mass % of carbon, 0.05-2.0 mass % silicon, 0.05-2.0 mass % manganese, and 0.0005-0.05 mass % rare earth metals, and, if necessary, one or more selected from suffer, calcium, aluminium, cobalt, chromium, molybdenum, niobium, boron, nickel, titanium, magnesium, zirconium, and nitrogen.
Additionally, U.S. Pat. No. 7,972,451 describes a pearlitic rail steel that is finished rolled between 850-1000° C. with the final pass imposing at least 6% area reduction ratio, wherein accelerated cooling of 2-20° C./s is applied to the rail web and accelerated cooling of 1-10° C./s is applied to the rail head and base to cool the rail from austenite to below 650° C. within 100 seconds of finish hot rolling. The rail processed as described must contain 200 or more pearlite blocks of 1-15 μm size within 0.2 mm2 area at a depth up to 10 mm below the rail surface. Furthermore, the carbon equivalent of the produced rail must exceed the number of proeutectoid cementite networks intersecting two 300 μm long perpendicular lines at the centerline of the rail web. The produced rail contains principally 0.65-1.4 mass % of carbon, 0.05-2.0 mass % of silicon, and 0.05-2.0 mass % of manganese, and further contains, as necessary, one or more of chromium, molybdenum, vanadium, niobium, boron, cobalt, copper, nickel, nitrogen, titanium, magnesium, calcium, aluminium, and zirconium.
Furthermore, U.S. Pat. No. 5,830,286 describes a pearlitic rail containing 0.85-1.2 mass % of carbon, 0.1-1.0 mass % silicon, 0.4-1.5 mass % manganese, and 0.0005-0.004 mass % boron, and further containing, as necessary, one or more of chromium, molybdenum, vanadium, niobium, and cobalt. The described rail must have a minimum hardness of 370 HV at 20 mm below the rail surface and a maximum variation in rail hardness of 30 HV within 20 mm of the rail surface.
Furthermore, U.S. Pat. No. 4,420,236 describes a pearlitic rail containing 0.65-0.85 mass % of carbon, 0.5-1.2 mass % of silicon, 0.5-1.2 mass % of manganese, 0.2-0.9 mass % of chromium, 0.005-0.05 mass % of aluminium, and 0.004-0.05 mass % of one or both niobium and titanium. The rail must also have a surface layer to a depth of 10 mm or more that is composed of fine pearlite with a tensile strength of 120 kg/mm2, a minimum reduction of area of 40%, and a hardness of 350 HV or more.
Moreover, U.S. Pat. No. 8,404,178 describes a pearlitic steel rail with a tensile strength of at least 1200 MPa that contains 0.6-1.0 mass % carbon, 0.1-1.5 mass % silicon, 0.4-2.0 mass % manganese, 0.035 mass % or less of phosphorous, 0.0005-0.0100 mass % sulfur, optionally 0.004 mass % or less of oxygen, optionally 0.001-0.01 mass % of calcium, no more than 2 ppm of hydrogen, and optionally one or more of vanadium, chromium, copper, nickel, niobium, molybdenum, or tungsten. The rail is also such that length of type A inclusions is 250 μm or less and the number of type A inclusions with a length of 1-250 μm is less than 25 per mm2 in the cross-section in the longitudinal direction of the rail head.
Additionally, U.S. Pat. No. 8,361,382 describes a pearlitic steel rail with a tensile strength of at least 1200 MPa that contains 0.6-1.0 mass % of carbon, 0.1-1.5 mass % of silicon, 0.4-2.0 mass % manganese, 0.035 mass % or less of phosphorous, 0.0100 mass % or less of suffer, 0.0010-0.010 mass % of calcium, and 0.004 mass % or less of oxygen. The steel rail can also contain one or more of vanadium, chromium, copper, nickel, niobium, molybdenum, and tungsten. The steel rail is also such that the length of type C inclusions is 50 μm or less and the number of type C inclusions with a length of 1-50 μm is 0.2-10 per mm2 in the cross-section in the longitudinal direction of the rail head.
Furthermore, U.S. Pat. No. 7,955,445 describes a pearlitic rail steel with a hardness of 380-480 HV to a depth of at least 25 mm in the rail head. The steel rail contains 0.73-0.85 mass % of carbon, 0.5-0.75 mass % of silicon, 0.3-1.0 mass % of manganese, 0.035 mass % or less of phosphorous, 0.0005-0.012 mass % of sulfur, and 0.2-1.3 mass % of chromium. The ratio of manganese to chromium in the steel rail is also within 0.3-1.0. According to the invention, the steel rail can also contain one or more of vanadium, copper, nickel, niobium, and molybdenum.
U.S. Pat. No. 8,241,442 describes the method of making a hypereutectoid, head-hardened steel rail that includes head hardening a steel rail having a composition containing 0.86-1.00 mass % carbon, 0.40-0.75 mass % manganese, 0.40-1.00 mass % silicon, 0.05-0.15 mass % vanadium, 0.015-0.030 mass % titanium, and sufficient nitrogen to react with the titanium to form titanium nitride. Furthermore, the patent specifies the range of cooling rates for the head hardening process in terms of coordinates on a plot of the temperature of the head of the steel rail versus cooling time. The upper bound of the cooling rate is defined by the line connecting (0 s, 775° C.), (20 s, 670° C.), and (110 s, 550° C.), and the lower bound of the cooling rate is defined by the line connecting (0 s, 750° C.), (20 s, 610° C.), and (110 s, 500° C.).
Finally, U.S. Patent Application Publication U.S. 2010/0186857 describes a pearlitic rail steel with a hardness of 380-480 HV to a depth of at least 25 mm in the rail head. The steel rail contains 0.73-0.85 mass % of carbon, 0.5-0.75 mass % of silicon, 0.3-1.0 mass % of manganese, 0.035 mass % or less of phosphorous, 0.0005-0.012 mass % of sulfur, 0.2-1.3 mass % of chromium, 0.005-0.12 mass % of vanadium, and 0.0015-0.006 mass % of nitrogen. The ratio of manganese to chromium in the steel rail is also within 0.3-1.0, and the vanadium to nitrogen ratio in the steel rail is within 8.0-30.0. According to the invention, the steel rail can also contain one or more of copper, nickel, niobium, and molybdenum.
Japanese Patent Publication 2002-030341 describes a low strength steel rail with a hardness of 220-300 HB that contains 0.60-0.95 mass % of carbon, 0.10-1.20 mass % of silicon, and 0.20-1.50 mass % of manganese, with allowances for one or more of 0.01-0.50 mass % of chromium, 0.01-0.2 mass % of molybdenum, and 0.1-2.0 mass % of cobalt. The steel rail can also contain one or more of copper, nickel, vanadium, niobium, and titanium, as necessary. The steel rail must also have a carbon equivalent between 0.6-1.0 and receive accelerated cooling of 1 to 2.5° C./s in the 800-500° C. range.
Japanese Patent Publication 2001-152290 describes a low strength steel rail with a hardness of 220-300 HB that contains 0.60-1.20 mass % of carbon, less than 0.2 mass % of silicon, and less than 0.4 mass % of manganese, with a further allowance of 0.01-0.20 mass % of chromium, as long as the sum of silicon, manganese, and chromium is less than 0.5 mass %. The steel rail can also contain one or more of molybdenum, copper, nickel, niobium, vanadium, titanium, and cobalt, as necessary.
Japanese Patent Publication 11350075 describes a pearlitic steel rail containing 0.60-1.20 mass % of carbon, 0.10-0.50 mass % of silicon, 0.30-1.20 mass % of manganese, and 0.0060-0.0200 mass % of nitrogen, with further allowances of one or more of chromium, molybdenum, copper, nickel, niobium, vanadium, cobalt, titanium, and boron.
Japanese Patent Publication 09316598 describes a steel rail containing 0.85-1.20 mass % of carbon, 0.10-1.00 mass % of silicon, 0.20-1.50 mass % of manganese, and 0.05-1.00 mass % of chromium, with further allowances for molybdenum, vanadium, niobium, cobalt, and boron. The steel rail must also possess a hardness of 320 HV or more at a depth of 20 mm and the difference in hardness between the base metal and the weld joint is restricted to 30 HV or less.
Japanese Patent Publication 2010-185106 describes a steel rail containing 0.50-1.0 mass % of carbon, 0.1-1.0 mass % of silicon, 0.1-1.5 mass % of manganese, less than 0.030 mass % of phosphorous, less than 0.020 mass % of sulfur, less than 0.005 mass % of aluminium, 0.25-1.5 mass % of chromium, and less than 0.0020 mass % oxygen, with additional allowances for nickel, molybdenum, and copper, as necessary. The rail also has a calculated specific resistance in the range of 21-24 μΩ cm and a fatigue crack propagation velocity below 2.5×10−8 m/cycle at a stress intensity factor of ΔK=15 MPa m1/2 at a depth of 10 mm in the rail where the pearlite interlamellar spacing is between 0.08-0.25 μm.
It is an object of this invention to provide improved alloy compositions for steel rail products, particularly for high carbon steel rail products with microstructures comprised principally of pearlite.
It is another object of this invention to provide an improved process of manufacturing steel rail products.
It is an aspect of this invention to provide a high carbon steel rail with enhanced ductility comprising 0.65-1.4 mass % of carbon, 0.1-1.5 mass % of silicon, 0.01-0.3 mass % of manganese, 0.1-1.5 mass % of chromium, and 0.005-0.05 mass % of titanium, with the remainder being iron and the unavoidable impurities. In one embodiment, the carbon content is from 0.65-0.75 mass % to promote intermediate strength and enhanced ductility. In another embodiment, the carbon, content is from 0.75-0.85 mass % to promote high strength and enhanced ductility. In yet another embodiment, the carbon content is from 0.85-1.0 mass % to promote even higher strength and enhanced ductility. In still another embodiment, the carbon content is from 1.0-1.2 mass % to promote even higher strength and enhanced ductility.
It is another aspect of this invention to provide a high carbon steel rail with enhanced ductility comprising 0.65-1.4 mass % of carbon, 0.5-1.5 mass % of silicon, 0.01-0.4 mass % of manganese, 0.1-1.5 mass % of chromium, and 0.005-0.05 mass % of titanium, with the remainder being iron and the unavoidable impurities. In one embodiment, the carbon content is from 0.65-0.75 mass % to promote intermediate strength and enhanced ductility. In another embodiment, the carbon content is from 0.75-0.85 mass % to promote high strength and enhanced ductility. In yet another embodiment, the carbon content is from 0.85-1.0 mass % to promote even higher strength and enhanced ductility. In still another embodiment, the carbon content is from 1.0-1.2 mass % to promote even higher strength and enhanced ductility.
It is another aspect of this invention to incorporate the addition of 0.005-0.05 mass % of titanium to a high carbon steel rail with manganese contents limited to 0.30 mass % or 0.40 mass %, since prior implementations of titanium in high carbon rail steels applied to higher manganese contents of 0.8-1.2 mass % (U.S. Pat. Nos. 7,288,159 and 7,217,329).
It is another aspect of this invention to produce a high carbon steel rail with a microstructure comprising at least 90% pearlite at a depth of between 2-20 mm below the rail head surface.
It is another aspect of this invention to produce a high carbon steel rail with a rail head surface hardness of at least 325 HB (Brinell hardness) for improved wear resistance.
It is another aspect of this invention to provide a high carbon steel rail as described above that further includes up to 0.5 mass % Mo, up to 0.05 mass % Nb, up to 0.3 mass % V, up to 1.0 mass % Cu, up to 1.0 mass % Ni, up to 1.0 mass % Co, up to 0.005 mass % B, up to 0.025 mass % N, up to 0.02 mass % Ca, up to 0.02 mass % Mg, up to 0.2 mass % Zr, up to 1.0 mass % Al, and/or up to 1.0 mass % W. These additional elements can be present in rail steel products for various reasons, but do not affect the novelty of steel rail compositions described above.
Finally, it is an aspect of this invention to provide the method of manufacturing the high carbon steel rail comprising the above compositions and characteristics, where the manufacturing process consists of the steps of: forming a rail shape by rolling of an austenitic structure; cooling of the austenitic structure of the whole rail or any portion of the rail to below the pearlite transformation temperature at a cooling rate sufficient to achieve a hardness of at least 325 HB on the surface of the rail head while generating a microstructure comprising at least 90% pearlite at a depth of between 2-20 mm below the rail head surface, where the austenite structure prior to pearlite transformation is either the austenite structure present after the rolling process or an austenite structure developed by reheating a cooled rail to above the austenite formation temperature, and the cooling is achieved either through ambient cooling and/or accelerated cooling comprising spraying, immersing, and/or flowing a cooling media across the entire surface, or any portion of the surface of the rail; and further cooling the rail to ambient temperature.
These and other objects and features shall now be described in relation to the following drawings and description.